This invention relates to marine propulsion devices and, more particularly, to marine propulsion devices, such as outboard motors, stern drive units and the like, including a shroud surrounding the propeller for augmenting propeller thrust and/or guarding the propeller against underwater obstructions.
It is known that the thrust delivered by propeller-driven marine propulsion devices can be increased by employing a so-called Kort-type shroud or nozzle which surrounds the propeller and defines a venturi-like flow passage for water. For maximum efficiency the outside shape of such a nozzle should not be greater in diameter than the opening at the entrance or mouth of the nozzle. In a nozzle designed for relatively high advance speeds this would require a thin nozzle section which may not have adequate structural strength to sustain all the loads imposed on it. Also, the aft or rear section of the nozzle should ideally taper to a thin trailing edge. However, nozzles having such a shape are vulnerable to damage upon striking underwater obstructions. As a practical compromise it usually is necessary to use a nozzle with a section which is thicker and more blunt-ended even though it results in greater fluid-dynamic drag.
Examples of Kort-type nozzle arrangements for propeller-driven marine propulsion devices are disclosed in the U.S. Kort Pat. No. 2,030,375, issued Feb. 11, 1936, the U.S. Anthes et al Pat. No. 3,499,412, issued Mar. 10, 1970 and the U.S. Hannan Pat. No. 3,508,517, issued Apr. 28, 1970. Examples of jet propelled marine propulsion devices including similar nozzle arrangements are disclosed in the U.S. Irgens Pat. No. 3,249,083, issued May 3, 1966 and the U.S. Stubblefield Pat. No. 3,494,320, issued Feb. 10, 1970. Attention is also directed to the U.S. Broadwell Pat. No. 3,149,605, issued Sept. 22, 1964, which discloses a propeller-surrounding guard arranged to also serve as a siphon for pumping water from the bottom or bilge of a boat.